In high quality large-screen television displays, the visibility of the scanning line structure can be reduced by scan line multiplication, e.g. line-doubling. In the well known standard NTSC system, alternate 60 Hz fields having 262.5 lines are interlaced 2:1 to provide 525 line vertical resolution in 30 Hz frames; conventional interlaced scanning has proven to be susceptible to motion artifacts and other anomalies at the 30 Hz frame rate, especially with large screen displays.
By doubling the scan rate, the scanning can be made non-interlaced, i.e. progressively scanned, and the frame rate doubled to 60 Hz so that each field constitutes a complete 525 line frame. Additional integer multiplication can be utilized to further disguise the scanning line structure. Such line multiplication tends to accentuate the motion deficiencies inherent in the original interlaced format, e.g. strobe effects, shimmer and jaggedness along the edges of moving objects; thus further processing is required to obtain smoother displayed motion through timing correction.
Such correction typically involves separating the luminance (Y) from the chrominance (C) with a comb filter and motion-processing at least the luminance portion utilizing circuitry including delay means to provide successive fields of Y data for comparison and interpolation. Integrated circuit application data published by NEC Corporation, Japan describes Y/C separation and motion detection in connection with IC product uPD9380 and describes Y-C interpolation in connection with uPD9382. Scan conversion including Y signal separation and data interpolation techniques are described in connection with uPD41101/uPD41102 digital delay lines.
Transfer of film to videotape has been performed for many years using an artifice known as pulldown, where a difference between the film frame rate and a higher video frame rate is reconciled by using successive film frames for two and three video fields respectively. Film with a frame rate of 24 per second is transferred to NTSC video (60 interlaced 262.5 line fields/30 525 line frames per second nominal) using 3/2 pulldown: every odd film frame is encoded in two fields, i.e. shown twice, and every even frame is encoded in three fields, i.e. shown three times. Without special compensation, there will be unwanted anomalies appearing on edges of moving images.
With line multiplication, the motion-processing required for film-sourced video is substantially different than that required for normal video (e.g. video camera) sources, so generally two separate motion-processors are required. Thus a line-doubler to be used with a mixture of standard video material and film-originated material needs to be able to operate in either of two motion-processing modes: a normal video mode and a film mode. Selecting between these two modes requires motion-detection circuitry that can quickly recognize the beginning and end of film material inserts and control automatic film mode entry and exit.; however, in known art, film-mode entry is prolonged excessively due to the time required for the film-mode processor to acquire synchronization after film-sourcing has been detected.
High quality doubled video display requires the reaction time period required to accomplish such mode-detection and mode-switching needs to be made as short as possible for both entry and exit in order to suppress motion-artifacts appearing at moving image edges during the transition periods. This is particularly important when dealing with source material that was originally transferred from film to video in a pull-down process then edited electronically such that the film/video field pattern is highly fragmented and spliced, often in a non-synchronous manner. Film-video mode detectors and controllers in line multiplication systems of known art fail to react and re-synchronize quickly enough, and thus show shimmering and raggedness at the edges of moving objects during film mode entry and exit transitions.